How different are your 3 tactics? And how often do you mix it up even when on winning runs?
I think I've found that if you stick with 1 winning formula too consistently FM find a way to play around it. And then as we know once 1 shaky result comes generally a couple more and a couple more after that.
But if you have 2 similar tactics with slight differences and mix them up every 2-3 games they struggle to work this out. Then having a 3rd completely different for when it looks like the ******* is coming you go to that at HT in the same you're struggling in and it recovers results. I'm playing a 3-5-2 and 3-4-3 which are attacking and direct and overall pretty similar. I've then got a more possession based 4-2-3-1 which I can throw in every now and then.
I'm currently in March with Inter and apart from a 1st leg fully rotated loss to Rangers I am undefeated since the end of August when I lost on MD2 against Roma 0-1. 23-5-1 in the league. Bizzarely 4 of my 5 draws are 0-0. So basically in 23 of the 24 games I've scored in the league I've won.
RIP your unbeaten streak.
I'm loathed to change tactics but normally will do it around Christmas then change it back for the run home. I find that if I change it too much it has the opposite effect and for whatever reason the team just tanks no matter how adept they are to the changed tactic.